In the field of silver halide photographic processes, a photographic process capable of obtaining positive images without forming negative images or carrying out intermediate treatment for negative images is called a direct positive photographic process. Photographic sensitive materials and emulsions used for such photographic processes are called direct positive sensitive materials and direct positive photographic emulsions, respectively.
Although there are many kinds of direct positive photographic processes, a process which comprises exposing previously fogged silver halide particles to light in the presence of a desensitizer and thereafter developing and a process which comprises exposing a silver halide emulsion having sensitive nuclei in principally the inner part of silver halide particles to light and thereafter developing in the presence of a fogging agent are most useful. The present invention relates to the latter. The silver halide emulsion having sensitive nuclei in principally the inner part of silver halide particles, which forms latent images in principally the inner part of particles is called an inner latent image type silver halide emulsion which is distinguished from a silver halide emulsion forming latent images on the surface of particles.
A process for obtaining direct positive images which comprises carrying out surface development of the inner latent image type silver halide photographic emulsion in the presence of a fogging agent and photographic emulsions and photographic sensitive materials used for such a process are described in U.S. Pat. Nos. 2,456,953, 2,497,875, 2,497,876, 2,588,982, 2,592,250, 2,675,318, 3,227,552, and 3,761,276, British Pat. Nos. 1,011,062 and 1,151,363, and Japanese Patent Publication No. 29405/68, etc.
In the above-described process for obtaining direct positive images, the fogging agent may be added to a developing solution. However, if the fogging agent is added to a photographic emulsion layer or another layer in the sensitive material so as to be adsorbed on the surface of silver halide particles, better reversal property can be obtained.
Examples of useful fogging agents used in the above-described processes for obtaining direct positive images include hydrazine and derivatives thereof described in U.S. Pat. Nos. 2,563,785, 2,588,982 and 3,227,552. Particularly, in U.S. Pat. No. 3,227,552, it has been described that hydrazides which are hydrazine derivatives and hydrazine compounds can be used by adding them not only to a developing solution but also to a sensitive layer.
Further, heterocyclic quaternary salt compounds described in U.S. Pat. Nos. 3,615,615, 3,719,494, 3,734,738 and 3,759,901 and Japanese Patent Application (OPI) Nos. 3426/77 and 69613/77 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") are known to be useful as fogging agents.
Moreover, the use of acylhydrazinophenylthiourea compounds is described in U.S. Pat. Nos. 4,030,925 (corresponding to German Patent Application (OLS) No. 2,635,316) and 4,031,127 (corresponding to German Patent Application (OLS) No. 2,635,317).
However, these known fogging agents are not desirable because of their bad influence upon preservability of the direct positive sensitive materials, they have an insufficient ability with respect to fogging the inner latent image type silver halide having a small particle size, the reversal property varies greatly due to variations in the bromine ion concentration of the developing solution, and the reversal property varies greatly due to variations in the amount of the fogging agent added.
On the other hand, direct positive images obtained by using a core/shell type emulsion together with a fogging agent represented by the general formula (I) as described later are not desirable because their images are inferior in sharpness and edge effect to form the so-called loose images, the reason of which is not clear. Further, the process is not desirable because its application is restricted, though it is the process by which high speed B/W direct positive images are now generally obtained.